my take on it is that if there is a film left by the blade, there there will be less friction, therefore slippage
if there is nothing but glass then friction, drag
squeak = friction
therefore 'squeaky clean' is a drag!
![Grin ;D](http://www.cleanitup.co.uk/smf/Smileys/classic/grin.gif)
an analogy is before your wife gets into the shower her skin is a bit oily and therefore slippy, when she gets out you find your hand sticks rather than slides!!!!
![Wink ;)](http://www.cleanitup.co.uk/smf/Smileys/classic/wink.gif)